The present invention relates to an incremental length or angle measuring system of the type which includes a measuring scale which defines an incremental graduation and at least one reference mark, a scanning unit which comprises a plurality of scanning fields for scanning the measuring scale, and an evaluating system for evaluating scanning signals generated by the scanning unit.
Length and angle measuring systems are known to the art, as well as methods for determining reference positions in such measuring systems. In one prior art approach, for example, machine or measuring system components which are movable relative to one another are driven from a starting position to a reference mark in order to determine and store the distance between the starting position and the reference mark, or to utilize the reference position defined by the reference mark as a zero point. Such a method can be used in an incremental length or angle measuring system such as described in German Pat. DE-PS No. 19 64 381. This method requires, however, unimpeded relative movability of the objects to be measured, since the components of the measuring system are firmly joined with the objects to be measured and must therefore be shifted in unison with the components of the measuring system up to the reference mark.
German DE-OS No. 16 73 887 discloses another prior art measuring system for a machine, which allows a reference position to be determined even when the slide piece of the machine is clamped fast with respect to the machine bed. In this system, the slide piece must first be driven into the position that is later to be used as the zero reference position. After this has been done, the slide piece is clamped fast to the machine bed. Then the scanning plate is moved relative to the scale until a reference mark is detected. Upon detection of the reference mark, the electronic counter of the measuring system is set to zero. The clamping of the slide to the bed of the machine can then be released and the slide piece driven to its desired position. The position of the reference mark, therefore, defines the reference position for further operations.
The prior art methods described above for determining a reference position as a starting position (prior to the initiation of working operations proper) are often not suitable for use with incremental measuring systems once working operations have commenced. For example, when working operations in progress are interrupted, it may not be possible to move machine components coupled to a measuring system to a desired position. For example, an automatic handler such as an industrial robot may be interrupted during its operation through a power failure. The robot then remains standing in its momentary position. The measuring value, determined as discussed above with reference to its original reference position, is lost through the power failure since the measurement was interrupted.
In order to continue the interrupted working operation, however, the reference position must be redetermined. A return movement of the robot from its momentary position into the original starting position is, however, as a rule not possible. For example, a tool positioned by the robot may be in engagement with the workpiece, thereby preventing relative movement between the tool and the workpiece.